Many providers of electric and telephone service (hereinafter, "utilities") often bury their pipes or cables (hereinafter, "conveyances") underground both for reasons of safety and esthetics. During the burial process, the utility typically records both the depth and location of the conveyance so that others seeking to excavate in the same general vicinity can locate the conveyance, thereby avoiding damage during such excavation. At present, utilities that bury conveyances record the location of each such conveyance relative to existing physical landmarks, such as roads, buildings, or bridges for example. From knowledge of the recorded distance of a conveyance from a landmark, a contractor seeking to excavate in the general vicinity can locate the buried utility conveyance and avoid causing any damage during excavation.
Unfortunately, utilizing a physical landmark as a reference point for locating an underground utility conveyance does not always facilitate accurate location of the conveyance. Occasionally, physical landmarks undergo a change. For example, a road may undergo widening or repair that may alter its relative distance to the buried utility conveyance. A building may undergo renovation or even demolition, precluding the ability to utilize such a structure as a point of reference for a conveyance location measurement.
Given that burial records may not always yield an accurate indication of the location of an underground utility conveyance, most utilities must physically locate their buried conveyances in order to effect a repair as well as to provide warnings to excavating contractors. In practice, a technician from the utility physically locates a buried utility conveyance using a radio signal detector to detect a locating signal (typically, an alternating current signal) impressed on a conveyance of interest. The detector utilized by the technician typically includes one or more frequency-sensitive magnetometers for detecting the locating signal radiated by the buried conveyance. As a technician displaces the radio signal detector in the general vicinity of the buried conveyance, a pair of horizontally-mounted the magnetometers in the detector will measure a peak value of the locate signal when the technician is directly above the cable, assuming that the conveyance radiates the locating signal equally in a circular pattern.
Some radio signal detectors also include a vertically mounted, frequency-sensitive magnetometer that will indicate the locating signal on opposite of the conveyance, and a null indication when the detector is directly above the conveyance. Generally, the null indication provided by the vertically mounted magnetometer will correspond to the peak indication provided by the horizontally mounted magnetometer when the radio signal detector is directly above the conveyance of interest. Thus, the technician can establish the location of the conveyance of interest by displacing the detector until the peak signal from the horizontally mounted magnetometers coincides with the null signal from the vertically mounted magnetometer.
The above-described locating method works well when no other buried utility conveyances are present in the general vicinity of the conveyance of interest. However, other buried utility conveyances are often present in the same general area as the conveyance of interest and each such neighboring conveyance will radiate its own locating signal. These locating signals can interfere with each other. Consequently, a technician trying to locate a particular buried utility conveyance of interest will some times detect the peak and null signals at different locations, rather than the same location above the buried conveyance of interest. The interference between the locating signals of neighboring buried conveyances has led to mis-location of buried conveyances, and resultant conveyance failures as a result of such mis-location.
Thus, there is a need for a technique for improving the accuracy of buried utility conveyance location by radio signal detection.